Of all the unanswered questions since the Boston Marathon bombing, the biggest is this: Why has al-Qaeda not carried out more attacks like the one in Boston?

In the past 12 years, al-Qaeda has repeatedly attempted attacks intended to match or exceed 9/11 in scope and scale. It set in motion plots to fly hijacked planes into the Library Tower in Los Angeles, Heathrow Airport and buildings in downtown London. It tried to blow up seven passenger planes carrying at least 1,500 passengers over the Atlantic Ocean using liquid explosive hidden in sports drinks. Its Yemeni affiliate tried to blow up Northwest Airlines Flight 253 over Detroit using abomb hidden in underwear. And it tried to blow up planes headed for the United States using bombs hidden in printer cartridges timed to explode over the eastern seaboard.

Thankfully, none of these attacks were successful. In Detroit, we averted disaster by pure luck — al-Qaeda succeeded in penetrating our defenses and getting a bomb onto a plane, but the bomb malfunctioned. But in other cases, the plots were foiled thanks to incredible intelligence work. The fact is, since 9/11, we have made it much harder for terrorists to carry out attacks on the scale of that day.

So why does al-Qaeda keep trying? Why does it remain so intently focused on spectacular, mass casualty attacks, instead of smaller scale attacks like Boston — attacks that are much easier to pull off, and much harder for us to detect and prevent?

Smaller scale attacks could be equally effective in paralyzing the country. Think about how a man and a teenage boy terrorized the Washington area with a series of sniper attacks in October 2002. It would be easy for al-Qaeda to carry out similar attacks in Washington, New York and other cities in the United States — assassinating members of Congress or innocent civilians, or setting off improvised explosive devices in major metropolitan areas.

Imagine the impact on our country if simultaneous bombs went off one Saturday afternoon at Tysons Corner Center and the Mall of America? Or if suicide bombers blew themselves up at several NFL games on live television? Or if backpack bombs like the one in Boston started going off again and again over the course of several days or weeks in different U.S. cities? Al-Qaeda could turn Washington, New York and Boston into Baghdad.

Yet for the past decade, al-Qaeda has not taken this route. Why?

One possible explanation is that the severity of the 9/11 attacks has worked in our favor. In striking the twin towers and the Pentagon and killing nearly 3,000 people, al-Qaeda set an extremely high bar for itself. The group may have feared that if it followed 9/11 with a series of small-scale attacks, it would been seen in the jihadist world as a sign of weakness — a concession that it could no longer pull off spectacular attacks on the scale of 9/11.

Perhaps, in the wake of the killing of Osama bin Laden and other high-profile blows , al- Qaeda leaders have become less concerned with such questions. Bin Laden reportedly kept pushing for spectacular attacks on symbolic targets until the very end. But now bin Laden is gone. Al-Qaeda’s strategic modus operandi could shift in his absence — especially as its affiliates in Yemen, East Africa and the Islamic Maghreb assume greater autonomy from al-Qaeda’s central leadership in Pakistan.

We still do not know if the two Chechen terrorists allegedly behind the Boston Marathon bombing were trained or deployed by al-Qaeda. But if al-Qaeda or its affiliates do turn out to be responsible for the Boston Marathon bombing, the question will become: Is Boston a turning point? Has al-Qaeda internally made a decision to focus less on replicating 9/11 and more on carrying out more frequent, lower-level attacks? Could Boston be the first of many?

Even if Boston does not turn out to be an al-Qaeda plot, the demonstration effect could spark a shift in strategy. Watching the massive news coverage of the Boston bombing, seeing how two kids with backpack bombs seem to have succeeded in putting a major U.S. city on lockdown, it may now dawn on al-Qaeda leaders that a series of small-scale attacks like this can have the same impact as one spectacular mass casualty attack.

If it does, that would be very bad news for America. Because it is impossible to defend in every place, at all times, against every possible form of attack. If Boston turns out to be not an outlier but a tipping point in the war on terror, the intelligence challenges will be enormous. We may no longer have the luxury of blowing up every high value terrorist we find with drones and vaporizing all the intelligence in their brains in the process. We might discover we need to interrogate live terrorists to learn what they know if we want to keep our country safe.

Though a combination of skill and luck, we’ve done well at preventing the next 9/11. Preventing the next Boston massacre might not be as easy.

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Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.